Development and design of active catalysts for the oxidation of formaldehyde present in wastewaters is of great importance. In this context, catalytic performance studies for oxidation of high formaldehyde containing solutions (1500 ppm) were carried out in a semibatch high-pressure reactor at 190-220degreesC and 15-35 bar of oxygen partial pressure. The removal efficiency of total organic carbon (TOC) was evaluated experimentally under different solid catalysts, using several heterogeneous composite oxides prepared in our laboratory (Mn/Ce, Co/Ce and Ag/Ce), as well as a commercial catalyst (CuO-ZnO/Al2O3). The Mn/Ce catalyst was the more active leading to high TOC conversions (99.4%). The TOC reductions were lower using Co/Ce, Ag/Ce and CuO-ZnO/Al2O3, respectively, 71.3%, 54.2% and 78.7%. Attempts were made to identify the influence of different Mn/Ce ratios. A high molar fraction of Mn leads to high TOC abatements. During the preparation procedure of catalysts the drying temperature does not affect significantly the catalytic area while increasing calcination temperature leads to lower surface areas. Carbon capacity adsorption was not observed for the Mn/Ce and CuO-ZnO/Al2O3 catalyst and leaching of the correspondent metals, Mn, Cu and Zn, was not significant. The catalytic wet oxidation (CWO) process was studied with an industrial high formaldehyde concentration effluent (800 ppm). Using an Mn/Ce catalyst the formaldehyde concentration decreased from 800 to 0.1 ppm and ammoniacal nitrogen from 420 to 155 ppm while 91.7% in TOC reduction was observed. These conversions were higher than those obtained with the commercial CuO-ZnO/Al2O3 catalyst. Therefore, Mn/Ce catalysts seem to be interesting options for the treatment of effluents from formaldehyde industry by CWO process. (C) 2003 Elsevier Science Ltd. All rights reserved.